Citation with minor modifications. Cannot explain it better.
This test is used to determine if the pixel at the current location in the framebuffer is currently owned by the GL context. If it is not, the window system decides the fate the incoming fragment. Possible results are that the fragment is discarded or that some subset of the subsequent per-fragment operations are applied to the fragment. This test allows the window system to control the GL’s behavior, for instance, when a GL window is obscured.
If the draw framebuffer is a framebuffer object, the pixel ownership test always passes, since the pixels of framebuffer objects are owned by the GL, not the window system. If the draw framebuffer is the default framebuffer, the window system controls pixel ownership.